生成目标代码的LLVM通行证存在分段错误

Question

我正在使用LLVM C++API编写一个C子集编译器，并使用一个生成目标代码的过程。但是，当源文件包含if-else块时，目标代码生成步骤会给出segfault。以下是我正在使用的代码

用于生成if-else块的编译器代码

llvm::Value *Conditional::generateCode(CodeKit &kit) {
auto *cond = condition->generateCode(kit);
if (cond == nullptr) {
return nullptr;
}
llvm::Function *func = kit.builder.GetInsertBlock()->getParent();
auto *ifBlock = llvm::BasicBlock::Create(kit.context, "if", func);
auto *elseBlock = llvm::BasicBlock::Create(kit.context, "else");
auto *mergeBlock = llvm::BasicBlock::Create(kit.context, "ifelsemerge");
kit.builder.CreateCondBr(cond, ifBlock, elseBlock);
kit.builder.SetInsertPoint(ifBlock);
if (ifstmt != nullptr) {
kit.symbolTable.enterScope();
ifstmt->generateCode(kit);
kit.symbolTable.exitScope();
}
kit.builder.CreateBr(mergeBlock);
ifBlock = kit.builder.GetInsertBlock();
func->getBasicBlockList().push_back(elseBlock);
kit.builder.SetInsertPoint(elseBlock);
if (elsestmt != nullptr) {
kit.symbolTable.enterScope();
elsestmt->generateCode(kit);
kit.symbolTable.exitScope();
}
kit.builder.CreateBr(mergeBlock);
elseBlock = kit.builder.GetInsertBlock();
func->getBasicBlockList().push_back(mergeBlock);
kit.builder.SetInsertPoint(mergeBlock);
return nullptr;

用于从llvm模块生成目标代码的代码。

void emitCode(CodeKit &kit) {
kit.module.print(llvm::outs(), nullptr);
auto irFile = "output.bc";
error_code ec;
llvm::raw_fd_ostream irFileStream(irFile, ec, llvm::sys::fs::F_None);
llvm::WriteBitcodeToFile(kit.module, irFileStream);
irFileStream.flush();
auto targetTriple = llvm::sys::getDefaultTargetTriple();
llvm::InitializeAllTargetInfos();
llvm::InitializeAllTargets();
llvm::InitializeAllTargetMCs();
llvm::InitializeAllAsmParsers();
llvm::InitializeAllAsmPrinters();
string error;
auto target = llvm::TargetRegistry::lookupTarget(targetTriple, error);
auto cpu = "generic";
auto features = "";
llvm::TargetOptions opt;
auto rm = llvm::Optional<llvm::Reloc::Model>();
auto targetMachine =
target->createTargetMachine(targetTriple, cpu, features, opt, rm);
kit.module.setDataLayout(targetMachine->createDataLayout());
kit.module.setTargetTriple(targetTriple);
auto objectFile = "output.o";
llvm::raw_fd_ostream objectFileStream(objectFile, ec,
llvm::sys::fs::OF_None);
llvm::legacy::PassManager pass;
auto fileType = llvm::CGFT_ObjectFile;
targetMachine->addPassesToEmitFile(pass, objectFileStream, nullptr,
fileType);
pass.run(kit.module);
objectFileStream.flush();
}

我目前正在测试的源文件

int factorial(int n) {
if (n <= 0)
return 1;
else
return n * factorial(n - 1);
}

生成的IR代码

; ModuleID = 'bootleg c compiler'
source_filename = "bootleg c compiler"
define i32 @factorial(i32 %n) {
entry:
%n1 = alloca i32
store i32 %n, i32* %n1
%n2 = load i32, i32* %n1
%"Less or Equal" = icmp sle i32 %n2, 0
br i1 %"Less or Equal", label %if, label %else
if:                                               ; preds = %entry
ret i32 1
br label %ifelsemerge
else:                                             ; preds = %entry
%n3 = load i32, i32* %n1
%n4 = load i32, i32* %n1
%Subtract = sub i32 %n4, 1
%call = call i32 @factorial(i32 %Subtract)
%Multiply = mul i32 %n3, %call
ret i32 %Multiply
br label %ifelsemerge
ifelsemerge:                                      ; preds = %else, %if
}

segfault 的GDB堆栈跟踪

#0  0x00007ffff4386bc0 in llvm::Instruction::getNumSuccessors() const () from /lib/x86_64-linux-gnu/libLLVM-10.so.1
#1  0x00007ffff4fd643c in llvm::BranchProbabilityInfo::computePostDominatedByUnreachable(llvm::Function const&, llvm::PostDominatorTree*) ()
from /lib/x86_64-linux-gnu/libLLVM-10.so.1
#2  0x00007ffff4fdb175 in llvm::BranchProbabilityInfo::calculate(llvm::Function const&, llvm::LoopInfo const&, llvm::TargetLibraryInfo const*) ()
from /lib/x86_64-linux-gnu/libLLVM-10.so.1
#3  0x00007ffff4fdb9c4 in llvm::BranchProbabilityInfoWrapperPass::runOnFunction(llvm::Function&) () from /lib/x86_64-linux-gnu/libLLVM-10.so.1
#4  0x00007ffff43a7d76 in llvm::FPPassManager::runOnFunction(llvm::Function&) () from /lib/x86_64-linux-gnu/libLLVM-10.so.1
#5  0x00007ffff43a7ff3 in llvm::FPPassManager::runOnModule(llvm::Module&) () from /lib/x86_64-linux-gnu/libLLVM-10.so.1
#6  0x00007ffff43a84a0 in llvm::legacy::PassManagerImpl::run(llvm::Module&) () from /lib/x86_64-linux-gnu/libLLVM-10.so.1
#7  0x0000555555565b73 in emitCode (kit=...) at c.ast.cpp:88
#8  0x0000555555579957 in generateCode (ast=...) at cc.cpp:25
#9  0x0000555555579a83 in main (argc=2, argv=0x7fffffffdfe8) at cc.cpp:41

Answer 1

ret i32 1
br label %ifelsemerge

是无效的IR，则在BasicBlock中必须只有一条终止符指令(例如br或ret(。您可以通过在插入分支之前检查终止符指令来避免这种情况

if (ifBlock->size() == 0 || !ifBlock->back().isTerminator()) {
kit.builder.CreateBr(mergeBlock);
}
// ...
if (elseBlock->size() == 0 || !elseBlock->back().isTerminator()) {
kit.builder.CreateBr(mergeBlock);
}

请注意，空块也是无效的，所以在条件的两个分支都退出函数的情况下，您可能不应该生成mergeBlock。AFAIK如果你没有分支到一个空的区块，可以把它留在里面。

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通常，您可以通过添加验证通行证或使用llvm::verifyFunction或llvm::verifyModule在前端捕获这些类型的错误。同样，使用生成的IR代码运行llc将给出更详细的错误消息。